It is known to compression mold hard (i.e., permanent) magnets, as well as soft (i.e., temporary) magnetic cores for electromagnetic devices (e.g., transformers, inductors, motors, generators, relays, etc.) from a plurality of ferromagnetic particles each encapsulated in a thermoplastic or thermosetting polymeric shell.
Soft magnetic cores are molded from ferromagnetic particles (i.e., less than about 1000 microns) such as iron, and certain silicon, aluminum, nickel, cobalt, etc., alloys thereof (hereafter generally referred to as iron), and serve to concentrate the magnetic flux induced therein from an external source (e.g., current flowing through an electrical coil wrapped thereabout). Unlike hard magnets, such cores, once magnetized, are very easily demagnetized, i.e., require only a slight coercive force (i.e., less than about 200 Oersteds) to remove the resultant magnetism. Ward et al. U.S. Pat. No. 5,211,896, for example, discloses one such soft magnetic core forming material wherein the polymeric shell comprises a thermoplastic polyetherimide, polyamideimide or polyethersulfone which, following molding, fuses together to (1) form a polymer matrix embedding the iron particles, and (2) so electrically insulate each iron particle from the next as to significantly reduce eddy current losses and hence total core losses (i.e., eddy current and hysteresis losses) in AC applications. Other possible matrix-forming thermoplastic polymers for this purpose are the polycarbonates and polyphenylene ethers among others known to those skilled in the art.
Permanent (i.e., hard) magnets are also known to be compression molded from such ferromagnetic particles as magnetic ferrites, rare-earth metal alloys (e.g., Sm--Co, Fe--Nd--B, etc.), and the like, and are subsequently permanently magnetized. Shain et al. U.S. Pat. No. 5,272,008, for example, discloses one such hard magnet-forming material comprising iron-neodymiumboron particles encapsulated in a composite polymeric shell comprising a thermosetting, matrix-forming, epoxy underlayer overcoated with a thermoplastic polystyrene outer layer. The polystyrene keeps the epoxy coated particles from sticking together before the epoxy is cured.
In Ward et al. U.S. Pat. No. 5,211,896 and Shain et al. U.S. Pat. No. 5,272,088, the shell-forming polymers are dissolved in an appropriate solvent, and a fluidized stream of the ferromagnetic particles spray-coated with the solution, using the co-called "Wurster" process. Wurster-type spray-coating equipment comprises a cylindrical outer vessel having a perforated floor through which a heated gas passes upwardly to heat and fluidize a batch of ferromagnetic particles therein. A concentric, open-ended, inner cylinder is suspended above the center of the perforated floor of the outer vessel. A spray nozzle centered beneath the inner cylinder sprays a solution of the shell-forming polymer, dissolved in a solvent, upwardly into the inner cylinder (i.e., the coating zone) as the fluidized ferromagnetic particles pass upwardly through the spray in the inner cylinder. The particles circulate upwardly through the center of the inner cylinder and downwardly between the inner and outer cylinders. The gas (e.g., air) that fluidizes the metal particles also serves to vaporize the solvent causing the dissolved shell-forming polymer to deposit as a film onto each particle's surface. After repeated passes through the coating zone in the inner cylinder, a sufficient thickness of polymer accumulates over the entire surface of each particle as to completely encapsulate such particle.
Rutz et al U.S. Pat. No. 5,198,137 mechanically blends or mixes boron nitride lubricant particles with polymer encapsulated particles prior to molding the particles into finished products to improve the flowability of the powder and the magnetic permeability of the molding, as well as to reduce the stripping and sliding die ejection pressures. Moreover, ethylene bisstearateamide lubricant particles--sold commercially under the trade name ACRAWAX.TM.), have heretofore been mixed/blended with polymer-encapsulated metal particles. Mechanical blending or mixing of the lubricant particles with the encapsulated particles, however, (1) can damage the polymer shell covering each of the metal particles, (2) does not uniformly distribute the lubricant particles throughout the particle mass, (3) results in a mass of loose particles having different densities and particle sizes, and a consequent propensity for segregation, and (4) adds additional cost to the preparation of the material.